This object indicates which port the receive clock may be selected as the system reference clock source. The value range starts at 1 and the maximum value is dependent upon the total number of physical ports on the specified CBR interface. Four and eight port CBR intefaces are supported. An error is returned if this object is set outside the range of the specified physical card interface. In addition, an error is returned if this slot has been selected as the system reference clock and an attempt is made to change to a port not in-service. The default value is port 1 (1).